There are mainly two situations when energy is consumed to maintain the indoor climate in a building; when heating or cooling is required. Heating of a building is done in situations when the outdoor temperature is lower than the desired indoor temperature, and when heating through absorption of heat radiation and other internal sources than the heating system is insufficient. Throughout this application heat radiation refers to electromagnetic radiation in the visible and solar ranges, i.e. wavelengths (λ) between 3×10−7 and 3×10−6 m. The main source of this heat radiation is of course the sun. This heat radiation is absorbed by the outside walls and the roof, as well as by the interior of the building, if the radiation passes through a window. Cooling, on the other hand, has to be done when the outdoor temperature is essentially higher than the desired indoor temperature, or when the combined effect of e.g. the outdoor temperature, heat radiation, and heating by other internal sources than the heating system, produces an indoor temperature that is higher than desired.
In large parts of the world, heating of buildings by intake of heat radiation through windows is a big problem, especially for buildings with large window areas. It is therefore of great interest to reduce this contribution to the heating by controlling this intake of heat radiation. On the other hand, such a system must not reduce the intake of daylight to such an extent that there is insufficient daylight for the persons being inside the building.
Today, the most frequently used techniques for variably reducing incoming heat radiation through windows include different types of mechanical shades, such as blinds, curtains and the like, that are either completely non-transparent, semi transparent or translucent.
In the near future “smart windows” may be commonly used in a large group of applications. Smart windows are particularly suitable for varying the intake of daylight as their transmittance of the whole or a part of the heat radiation range, may be continuously changed from transparent to non-transparent.
In this application a “smart window” represents any type of variably transmissive element integrated in a window pane, such as electrochromic elements, polymer dispersed liquid crystal elements (PDLC), dispersed particle elements, gasochromic and “light valves”. Further, a “variable radiation reducer” represents any system for reducing incoming heat radiation through windows, i.e. mechanical systems as well as smart windows. Still further, the term “non-transparent” will be used when referring to the state of lowest possible transmission for any variable radiation reducer, even though a particular variable radiation reducer does not reach a fully non-transparent state. Windows comprise vertical, inclined and horizontal glazings.
Known systems comprising smart windows, such as the systems presented in WO 96/13751, include thermal and/or radiation sensors for controlling the transmission through the windows. In these systems the transmission through the windows is controlled to achieve a predetermined temperature or radiation level, and the main goal is to save energy.
Smart windows are possible to combine with a solar cell and a battery, so that the windows become electrically self-supplying. In such a case a local control unit is arranged at the window. This control unit may then be connected to the sensors and an interface for setting the desired values for the temperature or radiation. This connection may either be of a wired or wireless type. A system of this kind may further incorporate one or more central control units, which coordinate several smart windows in a system.